1. Field of the Invention
The present invention relates to an apparatus for selectively receiving carrier wave signals, and more particularly, is directed to an improvement in a carrier wave signal receiving apparatus for selectively receiving frequency-modulated (FM) carrier wave signals, such as FM radio broadcasting signals, and obtaining a demodulated output signal based on the FM carrier wave signal received thereby.
2. Description of the Prior Art
In the field of super heterodyne receivers used for receiving FM carrier wave signals, such as FM radio broadcasting signals, there has been generalized employ a digital tuning system wherein, for example, a phase-locked loop (PLL) is utilized in place of an analog tuning system wherein a variable capacitor is used. In the super heterodyne receiver in which the digital tuning system is employed, the phase-locked loop is operative to set exactly various receiving frequencies, for example, under the control by a microcomputer, and manual adjustments to the receiving frequency by a user are not necessary.
In the super heterodyne receiver employing the digital tuning system, an automatic scanning tuning operation is performed. Under a condition in which the automatic scanning tuning operation is carried out, a receiving frequency provided for receiving selectively FM carrier wave signals is successively changed at predetermined regular frequency intervals by the PLL. Then, when there is an FM carrier wave signal which tunes with the receiving frequency, the change in the receiving frequency is temporarily ceased to keep the FM carrier wave signal tuning therewith, so that each FM carrier wave signal is searched. During the automatic scanning tuning operation in which such a signal search as mentioned above is carried out, it is desired to avoid such a situation that an image frequency signal which is not any FM carrier wave signal intended to be received acts as an FM carrier wave signal so that a malfunction by which the change in the receiving frequency is temporarily ceased is caused.
Accordingly, it has been proposed to cause an intermediate frequency signal which is obtained by frequency-converting the FM carrier wave signal to have a relatively high carrier frequency, for example, 10.7 MHz so that the image frequency signal is not located in a frequency range wherein the FM carrier wave signals expected to be selectively received exist and therefore cannot be detected in the automatic scanning tuning operation.
In addition, it is also proposed recently to form an intermediate frequency circuit portion of the super heterodyne receiver used for receiving the FM carrier wave signals, which includes an intermediate frequency filter for causing the intermediate frequency signal to pass therethrough, into an integrated circuit (IC) on the basis of the development in integrating technology related to electronic circuits. That is, in the super heterodyne receiver used for receiving the FM carrier wave signals, the intermediate frequency circuit portion is constituted by means of using one or more IC chips.
In such a case, the intermediate frequency filter contained in the IC chip is formed with resistive elements and capacitive elements which are materialized in the IC chip and therefore a passing frequency band of the intermediate frequency filter must be arranged to be located in relatively low frequency range. Therefore, in the intermediate frequency circuit portion including the intermediate frequency filter and comprising one or more IC chips, the intermediate frequency set therein is selected to be relatively low, for example, 150 kHz.
In the case where the intermediate frequency is selected to be relatively low, such as 150 kHz, in the super heterodyne receiver used for receiving the FM carrier wave signals as described above, the image frequency signal is undesirably located in the frequency range wherein the FM carrier wave signals expected to be selectively received exist. Consequently, it is feared that when a selected FM carrier wave signal is desired to tune with the receiving frequency successively changed by the PLL in the automatic scanning tuning operation, such a malfunction that an image frequency signal which results from another FM carrier wave signal adjacent to the selected FM carrier wave signal acts as the selected FM carrier wave signal to tune undesirably with the receiving frequency is brought about.